page_title: Polypodium (animal)
section_title: Description and life cycle
hierarchical_section_title: Polypodium (animal) / Description and life cycle
caption_attribution_description: English: Stolon stage of Polypodium hydriforme just after emerging from the host oocyte. Русский: Паразитическая стадия Polypodium hydriforme, только что покинувшая икринку-хозяина.
context_page_description: Polypodium is a genus of parasite attacking the eggs of sturgeon and similar fishes. It is one of the few metazoans that live inside the cells of other animals.
  Polypodium hydriforme is the only species of this monotypic genus. The parent family, order and class are also monotypic.
context_section_description: Polypodium hydriforme is an endocellular parasite with an unusual life cycle, a peculiar morphology, and high rates of DNA evolution. Polypodium spends most of its life inside the oocytes of acipenseriform fishes (sturgeons and paddlefish). Its hosts include Acipenser ruthenus, Polyodon spathula and Scaphirhynchus platorynchus. During this time, Polypodium develops from a binucleate cell into an inside-out planuliform larva and then into an elongate inside-out stolon; the epidermal cell layer is located internal to the body and the gastrodermis is located externally. This anatomy has a strong similarity with Placozoa. The embryo, larva and stolon are surrounded by a protective polyploid cell, which also functions in digestion. Just prior to host spawning, Polypodium everts to the normal position of cell layers, revealing tentacles scattered along the stolon. During eversion, the yolk of the host oocyte fills the gastral cavities of the parasite, supplying the future free-living stage with nutrients. Finally, upon emerging from the host egg in fresh water, the free-living stolon fragments into individual medusoid-like forms  that go on to multiply by means of longitudinal fission, form sexual organs, and ultimately infect host fish with their gametophores.
caption: Free living stolon.
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